1. Field of the Invention
The present invention relates to a voltage regulator which is capable of suppressing dispersion in an output short circuit current of the voltage regulator.
2. Description of the Related Art
FIG. 3 shows a circuit diagram of a conventional voltage regulator. The conventional voltage regulator includes: a voltage regulator control circuit having a reference voltage circuit 10, bleeder resistors 11 and 12 through which an output voltage Vout at an output terminal 6 is divided, and an error amplifier 13 for amplifying a difference between a reference voltage Vref1 and the division voltage; and an output P-channel MOS transistor 14. The conventional voltage regulator is operated with a voltage VDD1 supplied from a voltage source 15.
When an output voltage from the error amplifier 13 is assigned Verr, and a voltage at a node between the bleeder resistors 11 and 12 is assigned Va, if Vref1>Va, the output voltage Verr decreases, while if Vref1<Va, the output voltage Verr increases. That is, the voltage regulator control circuit operates to decrease an ON-resistance of the output P-channel MOS transistor 14 to increase the output voltage Vout when the output voltage becomes lower. Conversely, the voltage regulator control circuit operates to increase the ON-resistance of the output P-channel MOS transistor 14 to decrease the output voltage Vout when the output voltage Verr becomes high. Thus, the voltage regulator control circuit operates to hold the output voltage Vout at a constant value.
In general, in the case of the voltage regulator, since an output current is supplied from the output P-channel MOS transistor 14, when a load is lightened, a loss of the output P-channel MOS transistor 14 becomes extremely large. Thus, a voltage regulator as shown in FIG. 4 is designed in which a case where a load is short-circuited is taken into consideration.
The voltage regulator shown in FIG. 4 includes a current limiting circuit at its output terminal. A P-channel MOS transistor 21 is provided for the purpose of monitoring a drain current of the output P-channel MOS transistor 14, i.e., an output current. A W/L value (with W denoting width and L denoting length) of the P-channel MOS transistor 21 is set to be much smaller (e.g., 1/100) than that of the output P-channel MOS transistor 14. The output P-channel MOS transistor 14 and the P-channel MOS transistor 21 show a current mirror relation. Hence, when a load resistance decreases and thus the output current increases, a drain current of the P-channel MOS transistor 21 increases accordingly. As a result, an electric potential difference developed across mutually opposite terminals of a resistor 22 also increases. When the electric potential difference developed across the mutually opposite terminals of the resistor 22 reaches a threshold voltage of an N-channel MOS transistor 23, the N-channel MOS transistor 23 is turned ON. Thus, an invert circuit including the N-channel MOS transistor 23 and a resistor 24 turns ON a P-channel MOS transistor 25. As a result, since the control is carried out so that a gate to source voltage of the output P-channel MOS transistor 14 decreases, an output current is limited based on a negative feedback operation.
Moreover, the output current is limited at an operating point at which the electric potential difference developed across the mutually opposite terminals of the resistor 22 is considered equal to the threshold voltage of the N-channel MOS transistor 23. Here, a backgate bias voltage is applied to the N-channel MOS transistor 23. Hence, since the threshold voltage of the N-channel MOS transistor 23 decreases as the output voltage decreases, the value of the output current is limited to a low value. It is known that a relationship between the output current and the output voltage shows a foldback characteristics as shown in FIG. 5 (see JP Hei 4-195613 A (Page 3, FIG. 1)).
However, in the conventional voltage regulator as shown in FIG. 4, when the load is lightened, the output current is limited at the operating point at which the electric potential difference developed across the mutually opposite terminals of the resistor 22 is given as being equal to the threshold voltage of the N-channel MOS transistor 23. Hence, there arises a problem in that dispersion is generated in an output short circuit current due to an influence of the manufacturing dispersion in the threshold voltage of the N-channel MOS transistor 23 and the resistance value of the resistor 22, and thus it is difficult to control the output short circuit current to a set value. A loss of the output P-channel MOS transistor 14 causes the calorification. In this case, the loss of the output P-channel MOS transistor 14 is not permitted to exceed an allowable level. Consequently, it is desirable that the output short circuit current has a small value free from the dispersion.